A microwave resonator is essentially a tuned electromagnetic circuit which passes energy at or near a resonant frequency. It can be used as a filter to remove electromagnetic signals of unwanted frequencies from input signals and to ouput signals having a preselected bandwidth centered about one or more resonant frequencies.
The resonator comprises a generally tube-like body through which electromagnetic waves are transmitted. Typical shapes used for such resonators include cylinders, rectangular bodies, and spheres, although shape in itself is not a limitation of the present invention. The electromagnetic energy is typically introduced at one end by such means as capacitive or inductive coupling. The side walls of the resonator cavity act as a boundary which confine the waves to the enclosed space. In essence, the electromagnetic energy of the fields propagating through the waveguide are received at the downstream end by means of reflections against the walls of the cavity.
The resonant frequency associated with the waveguide is a function of the cavity's dimensions. Accordingly, a change in temperature causes the resonant frequency to change owing to expansion or contraction of the resonator material, which causes the effective dimensions of the cavity to change.
It has therefore been the practice to construct such resonators from relatively expensive temperature-stable materials such as an invar nickel-steel alloy (herein referred to as "invar steel"). Even the use of such materials, however, has not been a wholly acceptable solution to frequency shift. At 12 GHz, for example, it has been found that an invar steel resonator shifts 0.9 MHz over a typical communications satellite's operating temperature. In some applications, a shift of that magnitude is excessive and causes performance to be compromised.
Broadly, the present invention provides a temperature-compensating resonator for reducing such frequency shifts. Such resonator comprises a waveguide body having a cavity sized to maintain electromagnetic waves of one or more selected resonant frequencies, means for coupling electromagnetic energy into and out of the resonator, and temperature-compensating structure within the cavity configured to undergo temperature-induced dimensional changes which minimize the resonant frequency change that would otherwise be caused by the temperature-induced dimensional change of the waveguide cavity.
Even when a resonator made of invar steel or the like provides acceptable frequency stability in the face of temperature change, the use of such material presents disadvantages for some applications such as satellite communication.
First, invar steel is a relatively heavy material and is therefore disadvantageous where payload weight is an important factor. Second, invar steel, as well as other low thermal coefficient materials, possesses low thermal conductivity. In state of the art high-power communication satellites, a substantial amount of heat must be dissipated. In some cases, temperatures may be reached which can melt the steel. Invar's poor heat conductivity requires that active means for cooling the resonators be employed. Accordingly, additional weight and space must be dedicated to the cooling of these components; provision must be made for the size and weight associated with the cooling hardware and its associated power requirements.
Accordingly, in one form the present invention is directed to a cavity resonator particularly suitable for use in high-power communication satellites. The resonator comprises a body made of a relatively light weight, thermally conductive material that has heretofore been inappropriate for such applications because of associated high thermal expansion co-efficients. Such resonator includes temperature-compensation means for substantially offsetting temperature-induced changes in resonant frequency caused by dimensional changes in the cavity dimensions. In a preferred form this resonator utilizes a bimetallic temperature compensation means to accommodate the larger temperature-induced changes in the resonator cavity. Accordingly, such materials can be used which have advantages over invar steel. For example, lighter, more easily machined, higher conductivity metals such as aluminum can be used despite the fact that their temperature coefficients have heretofore limited their use.